Regarding material fatigue, modern motor-vehicle components, such as fuel-injection systems (direct gasoline injection, BDE; common rail) and hydraulic units for anti-lock braking systems (ABS), are increasingly no longer designed to be resistant to fatigue, but are rather designed to have a fatigue limit. The aim of the fatigue-limit design is that the relevant components reliably satisfy their specified service lives but are not resistant to fatigue, that is, they have a finite service life. This allows the components to be designed to be smaller, lighter, less expensive, and therefore more functional as well (that is, each can be brought into line with its exact function and operating conditions). A component failure must be reliably prevented, since this can generally lead to serious material damage and injury to persons (vehicle fire, brake failure, etc.).
A method and control unit of the type mentioned at the outset are referred to in German Patent Publication No. 195 16 481, which refers to a method and a control unit for monitoring, storing, and outputting influence parameters of the control unit. The monitored and stored data can be output as needed, for example, for assessing the probability of failure and reliability of the control unit. However, the fact that the influence parameters monitored and stored must first be fetched out of the control unit and input into an external analyzer device for determining the failure probability only allows a very irregular and rare determination of the failure probability.
According to the related art, one proceeds as follows for determining the failure probability of a component: First of all, a so-called fatigue-limit test is carried out for the components to be tested, that is, for the individual component parts of the components. In the course of the fatigue-limit test, the tolerable loading for the component or the component parts is determined until they fail. A so-called Woehler curve (stress-number curve) is then ascertained for some component parts of the components. The Woehler curve is an observation function (characteristic function) of the loading capacity of a component part. A so-called loading collective (or collective loading) is then determined, which is customary for the operation of the component or the component parts and, in the case of motor-vehicle components, includes, for example, x % city driving, y % inter-urban (overland) driving, and z % expressway driving, where x %+y %+z %=100%.
A collective damage sum (D_Koll), which is a measure of the accumulated damage of the component to be tested after a collective run-through (cycle), is determined according to the so-called linear damage-accumulation hypothesis of Palmgren-Miner, using the collective loading and the Woehler curve. Using the Woehler curve of the structural element and the fatigue-limit test, a tolerable-damage sum (D—50%), which is a measure of the damage sum that the component can endure up to failure, is likewise determined according to Palmgren-Miner. The failure probability of the component may be calculated by comparing collective-damage sum (D_Koll) and tolerable damage sum (D—50%) within the scope of a failure-probability calculation.
In other words, tests on real component parts of a component determine, first of all, which loading and which damage sum the component parts and the component may endure, respectively, up to the point of failure. Secondly, a selected, collective loading is run through during the operation of the component, and influence parameters are measured in the process. The sum of the damage, which acts on the component during the collective run-through of the selected, collective loading, is calculated by evaluating the influence parameters.
The component parts of the components may now be designed in such a manner, that collective-damage sum (D_Koll) lies below damage sum (D—50%) by a sufficiently large safety factor and the structural elements therefore do not fail during an average, estimated operational life. This type of component design may be referred to as fatigue-limit design.
One may not plot an arbitrary number of influence parameters due to the relatively limited storage space in a motor-vehicle control unit. This means that the collective loading must be selected carefully, in order to still supply a reliable indicator of the real operating conditions of the components in spite of the low number of influence parameters plotted.
According to the related art, the fatigue limit is established in the product-formation process. In this case, a customary procedure is the above-described method, the so-called nominal-stress concept (cf. Haibach, E.: Betriebsfestigkeit, Verfahren und Daten zur Bauteilberechnung [Fatigue Limit, Method and Data for Designing Component Parts], Duesseldorf: VDI Publishing House GmbH (1989), ISBN 3-18-400828-2, Chapter 3.2 Calculation of Service Life from the Nominal Stress). The nominal-stress concept requires the Woehler component-part test (single-stage test) (cf. Haibach, E.: Fatigue Limit . . . , at the specified location, Chapter 2.1 Woehler Tests), the fatigue-limit test (multistage test) (cf. Haibach, E.: Fatigue Limit . . . , at the specified location, Chapter 2.2.1 Working Load and Collective Loading), and the collective-loading measurement, in which the parameters relevant to damage are determined during practical use. Collective-damage sum (D_Koll) at or with respect to a selected design point (that is, service-life target, for example, length of travel, switching cycles, operating time, etc.) is calculated from the Woehler component-part test and the collective loading, using the Miner rule (cf. Miner, M. A.: Cumulative Damage in Fatigue, J. Appl. Mech. 12 (1945), pp. 159-164 and Haibach, E.: Fatigue Limit . . . , at the specified location, Chapter 3.2 Calculation of Service Life from the Nominal Stress). In addition, tolerable-damage sum (D—50%) resulting in the failure of the component is determined by a fatigue-limit test. In the scope of the fatigue-limit test, the component part is loaded to the point of failure, using a typical collective loading. The comparison of D—50% and D_Koll determines whether or not the design of the component parts or the component is reliable.
The collective loading, which represents a determining influence variable of the available method for establishing the fatigue limit, is user-specific and is essentially a function of the following influence parameters:                individual usage habits;        type and parameters of the open-loop control/closed-loop control of a function that may be implemented by the component;        measuring accuracy of sensors for controlling/regulating the function; and        design or structural characteristics.        
Using a fuel-injection system for an internal combustion engine as an example, these influence parameters include, in particular:                characteristic maps typical of the application;        design of a pressure regulator for regulating an injection pressure prevailing in the fuel-injection system;        dynamic pressure increases dependent on construction;        exceptional events (breakdown);        measuring accuracy of a pressure sensor situated in the fuel-injection system;        individual driving technique (sporty or comfort-oriented driver);        operating conditions of the vehicle (for example, delivery traffic);        number of starts of the internal combustion engine; and        total distance driven, among other things.        
To design the structural elements of the components, the available method must assume worst-case conditions for each influence parameter, since the exact, subsequent operating conditions vary individually. To design the structural elements, one must assume the most severe collective loading possible for the application in question. This is, of course, full of uncertainty. In order to reliably cover this, some conservative assumptions must be made regarding the layout of the collective loading, which makes it necessary for the component parts to have higher strengths. In essence, higher strengths may only be achieved by higher-quality, that is, more expensive materials, more expensive manufacturing methods, or designs that are more complicated structurally. Therefore, higher strengths result in the components being more expensive and having a higher weight.